U.S. patent application number 14/132429 was filed with the patent office on 2014-06-26 for optical frequency filter and a detector including such a filter.
This patent application is currently assigned to COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES. The applicant listed for this patent is COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES. Invention is credited to Roch ESPIAU DE LAMAESTRE.
Application Number | 20140175282 14/132429 |
Document ID | / |
Family ID | 48128471 |
Filed Date | 2014-06-26 |
United States Patent
Application |
20140175282 |
Kind Code |
A1 |
ESPIAU DE LAMAESTRE; Roch |
June 26, 2014 |
OPTICAL FREQUENCY FILTER AND A DETECTOR INCLUDING SUCH A FILTER
Abstract
An optical frequency filter comprises a support layer having
reflective elements formed thereon, the reflective elements
defining at least one periodic grid of substantially parallel
slits, the period P, the height, and the width of the slits being
selected in such a manner that the reflective elements form a
wavelength-selective structure for a wavelength lying in a
determined range of wavelengths. The support layer material has a
refractive index n.sub.h and includes inclusions of a material of
refractive index n.sub.b, where n.sub.b is strictly less than
n.sub.h. The inclusions are flush with the surface of the support
layer opposite from its surface on which the reflective elements
are formed, and present height h.sub.bwherein
0.5h.ltoreq.h.sub.b.ltoreq.1h.sub.h, h.sub.h being the support
layer height, and width l.sub.bwhere
0.05P.ltoreq.l.sub.b.ltoreq.0.75P. Each inclusion is situated at
least in part between two reflective elements.
Inventors: |
ESPIAU DE LAMAESTRE; Roch;
(Grenoble, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES
ALTERNATIVES |
Paris |
|
FR |
|
|
Assignee: |
COMMISSARIAT A L'ENERGIE ATOMIQUE
ET AUX ENERGIES ALTERNATIVES
Paris
FR
|
Family ID: |
48128471 |
Appl. No.: |
14/132429 |
Filed: |
December 18, 2013 |
Current U.S.
Class: |
250/338.1 ;
250/226; 359/568 |
Current CPC
Class: |
G02B 5/204 20130101;
G02B 5/20 20130101; H01L 31/02168 20130101; G02B 2006/12107
20130101; G02B 5/1809 20130101; G02B 5/1814 20130101; Y02E 10/50
20130101; G01J 5/0862 20130101; H01L 31/02327 20130101 |
Class at
Publication: |
250/338.1 ;
359/568; 250/226 |
International
Class: |
G02B 5/18 20060101
G02B005/18; G01J 5/08 20060101 G01J005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2012 |
FR |
12 62418 |
Claims
1. An optical frequency filter comprising a support layer having
reflective elements formed thereon, the reflective elements
defining at least one periodic grid of substantially parallel
slits, the period P, the height h, and the width l of the slits
being selected in such a manner that the reflective elements form a
wavelength-selective structure for a wavelength lying in a
determined range of wavelengths, wherein the support layer is made
of a material of refractive index n.sub.h and includes inclusions
of a material of refractive index n.sub.b, where n.sub.b is
strictly less than n.sub.h, these inclusions being flush with the
surface of the support layer opposite from its surface on which the
reflective elements are formed, and presenting a height h.sub.b
such that 0.5h.sub.h.ltoreq.h.sub.b.ltoreq.1h.sub.h, where h.sub.h
is the height of the support layer, and a width l.sub.b such that
0.05P.ltoreq.l.sub.b.ltoreq.0.75P, with each inclusion being
situated at least in part between two reflective elements.
2. An optical filter according to claim 1, wherein the difference
between the two refractive indices n.sub.h and n.sub.b is greater
than or equal to 0.25n.sub.h.
3. An optical filter according to claim 1, wherein the height
h.sub.h of the support layer is such that: 0.85 .lamda. 2 n h
.ltoreq. h h .ltoreq. 1.5 .lamda. 2 n h ##EQU00003## where .lamda.
is a wavelength in the predetermined range of wavelengths.
4. An optical filter according to claim 1, wherein the refractive
index n.sub.h is greater than or equal to (5/3)n.sub.max, where
n.sub.max is the highest refractive index of the surrounding
materials.
5. An optical filter according to claim 1, including a substrate,
with a layer made of a material having refractive index less than
n.sub.h being arranged between the support layer and the
substrate.
6. An optical filter according to claim 1, wherein the width of the
slits of said at least one periodic grating is less than one-third
the period P of the grating.
7. An optical filter according to claim 1, wherein the reflective
elements are made of metal and have a height lying between the skin
thickness of the metal and .lamda./10n, where n is the refractive
index of the material included in the slits and .lamda. is a
wavelength in the predetermined range of wavelengths.
8. An optical filter according to claim 1, wherein the height
h.sub.b of the inclusions is advantageously such that
0.5h.ltoreq.h.sub.b.ltoreq.0.95h.sub.h.
9. A detector for detecting electromagnetic radiation in a
predetermined range of wavelengths, the detector including a
detection circuit sensitive to said range of wavelengths, and
includes an optical filter according to claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to optical frequency filters
used in particular in the field of visible and infrared
photodetection. By way of example, they may be used for the
spectral detection of gases and for visible and infrared color
imaging.
BACKGROUND OF THE INVENTION
[0002] Optical filters are already known that comprise a support
layer having metal reflective elements formed thereon to define a
grating of slits smaller than the wavelength that is to be
filtered. The amplitude of transmission is adjusted both by the
width and by the thickness of the slits, with the thickness being
selected to be of the same order as but less than half the
wavelength under consideration.
[0003] That type of filter is described in particular in the
articles by J. A. Porto et al. "Transmission resonances on metallic
gratings with very narrow slits", Physical Review Letters, 83, p.
2845, 1999, and by G. Vincent et al. "Large-area dielectric and
metallic freestanding gratings for mid-infrared optical filtering
application", Journal of Vacuum Science Technology, B 26, p. 852,
2008.
[0004] Thus, those types of filter require thick reflective
elements that are sometimes suspended directly above air, so that
the refractive index of the support is as close as possible to that
of the incident medium (air/vacuum).
[0005] Under such conditions, such an optical filter is
technologically difficult to make.
[0006] Furthermore, the spectral range over which good rejection is
ensured around the main transmission peak is very limited.
[0007] Another optical filter is known that comprises reflective
elements forming a conventional grating of slits, a halfwave plate
supporting the grating and a medium in contact with the halfwave
plate and presenting relative thereto a refractive index contrast
that is as great as possible.
[0008] Such an optical filter is described in particular in
Document FR 2 959 021.
[0009] With such a filter, the halfwave plate forms a waveguide
under the grating of slits, the grating also being capable of
exciting plasmon surface modes. The combination of the grating, of
the plate, and of the large refractive index contrast makes it
possible to form an electromagnetic resonator that effectively
traps light in the plate before it escapes via the medium in
contact with the plate.
[0010] Furthermore, that optical filter is simpler to fabricate
than the suspended optical filter.
[0011] Nevertheless that filter presents certain drawbacks.
[0012] In particular, although its dimensions can be selected so as
to optimize transmission at a given wavelength, the transmission
spectrum of the filter presents significant peaks away from that
wavelength.
[0013] This leads to poor rejection outside a rather narrow
wavelength window.
[0014] That puts limitations on the use of the optical filter.
[0015] Thus, when the filter is used for hyperspectral imaging, the
spectral range of interest is limited. Interference can occur
between pixels of different colors, thereby complicating the color
reconstruction of the image.
[0016] Also, when used in a gas sensor, it is important to
distinguish spectrally between weak signals that are specific to
different species. Unfortunately, the limitations of the optical
filter lead to a degraded signal-to-noise ratio for detection.
OBJECT AND SUMMARY OF THE INVENTION
[0017] An object of the invention is to mitigate the drawbacks of
prior art optical filters by proposing an optical frequency filter
presenting very good rejection over a spectral range that is very
wide, good transmission over a narrow spectral range, and good
angular insensitivity, while being compact and simpler to
fabricate.
[0018] Thus, the invention provides an optical frequency filter
comprising a support layer having reflective elements formed
thereon, the reflective elements defining at least one periodic
grid of substantially parallel slits, the period P, the height h,
and the width l of the slits being selected in such a manner that
the reflective elements form a wavelength-selective structure for a
wavelength lying in a determined range of wavelengths.
[0019] According to the invention, the support layer is made of a
material of refractive index n.sub.h and includes inclusions of a
material of refractive index n.sub.b, where n.sub.b is strictly
less than n.sub.h, these inclusions being flush with the surface of
the support layer opposite from its surface on which the reflective
elements are formed, and presenting a height h.sub.b such that
0.5h.sub.h.ltoreq.h.sub.b.ltoreq.1h.sub.h, where h.sub.h is the
height of the support layer, and a width l.sub.b such that
0.05P.ltoreq.l.sub.b.gtoreq.0.75P, with each inclusion being
situated at least in part between two reflective elements.
[0020] The height h.sub.b of the inclusions is advantageously such
that 0.5h.sub.h.ltoreq.h.sub.b.ltoreq.0.95h.sub.h. Preferably, the
difference between the two refractive indices n.sub.h and n.sub.b
is greater than or equal to 0.25n.sub.h.
[0021] Likewise, that height h.sub.h of the support layer is such
that:
0.85 .lamda. 2 n h .ltoreq. h h .ltoreq. 1.5 .lamda. 2 n h
##EQU00001##
where .lamda. is a wavelength in the predetermined range of
wavelengths.
[0022] Preferably, the refractive index n.sub.h is greater than or
equal to (5/3)n.sub.max, where n.sub.max is the highest refractive
index of the surrounding materials.
[0023] Preferably, the optical filter is placed on a substrate with
a layer made of a material having refractive index less than
n.sub.h being arranged between the support layer and the
substrate.
[0024] Advantageously, the width of the slits of said at least one
periodic grating is less than one-third the period P of the
grating.
[0025] Furthermore, the reflective elements are advantageously made
of metal and have a height lying between the skin thickness of the
metal and .lamda./10n, where n is the refractive index of the
material included in the slits and .lamda. is a wavelength in the
predetermined range of wavelengths.
[0026] The invention also provides a detector for detecting
electromagnetic radiation in a predetermined range of wavelengths,
the detector including a detection circuit sensitive to said range
of wavelengths, and an optical filter of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The invention can be better understood and other objects,
advantages, and characteristics thereof appear more clearly on
reading the following description which is made with reference to
the accompanying drawings, in which:
[0028] FIG. 1 is a section view of a first embodiment of an optical
filter of the invention;
[0029] FIGS. 2a to 2f show different steps in a method of obtaining
a second embodiment of an optical filter of the invention;
[0030] FIGS. 3 to 5 show two transmission responses each, one for
an optical filter of the invention and the other for an optical
filter in accordance with the second embodiment;
[0031] FIG. 6 shows two transmission responses, one for an optical
filter of the invention and the other for a filter that is similar
but in which a doped semiconductor plate is inserted;
[0032] FIG. 7 is a map of the maximum value of a cost function F as
a function of the height and of the width of the inclusions in an
optical filter of the invention; and
[0033] FIG. 8 is a map of the resonance wavelength as a function of
the height and of the width of the inclusions in the filter of the
invention used for preparing FIG. 7.
MORE DETAILED DESCRIPTION
[0034] Elements that are shown in more than one of the figures are
given the same references in each of them.
[0035] FIG. 1 shows an example of an optical filter 1 of the
invention in section view.
[0036] The frequency filter 1 includes reflective elements 10 that
are formed on one face 110 of a support layer 11 made of dielectric
material.
[0037] The reflective elements 10 are separated by slits of width
l. These slits form at least one grating of substantially parallel
slits, the grating being periodic of period P.
[0038] Thus, the filter 1 may have only a single grating of
parallel slits, the elements 10 then forming lines or strips.
[0039] It is also possible for the filter 1 to have two gratings of
slits that are periodic and orthogonal. Under such circumstances,
the reflective element 10 would be in the form of rectangular
slabs.
[0040] The slits 100 go down to the layer 11. In other words, the
bottoms of the slits 100 are constituted by portions of the face
110 of the layer 11.
[0041] The period P, the width l, and the height h of the slits are
selected so that the reflective elements 10 form a structure that
is selective in transmission for a wavelength lying in a determined
range of wavelengths.
[0042] For selecting these parameters, reference may be made in
particular to the article by J. Le Perchec et al., Optics Express,
Vol. 19, Issue 17, pp. 15720-15731 (2011).
[0043] Thus, the width l of the slits should be selected to be less
than P/3. In general, the smaller the width l, the greater the
selectivity of the filter in wavelength.
[0044] Furthermore, the value of the period P is determined so as
to adjust the resonant wavelength on the selected wavelength of
interest.
[0045] In particular, the reflective elements 10 may be made of
metal, e.g. of aluminum, silver, gold, or copper.
[0046] They could also be made of a material capable of propagating
surface waves, such as an absorbent material. This material may
thus be silicon in the ultraviolet or heavily n- or p-doped silicon
in the infrared.
[0047] The height h of the reflective elements 10 or of the slits
100 is preferably the same for all of the elements 10.
[0048] In particular, it lies between the thickness of the skin of
the metal and about .lamda./10n, where n is the refractive index of
the material included in the slits, specifically air. By way of
example, the height of a reflective element made of aluminum may
lie in the range approximately 15 nanometers (nm) to 500 nm
depending on whether the wavelength is in the visible or the
infrared. For example, it would be 25 nm for a reflective element
made of gold for use at a wavelength of 4 micrometers (.mu.m).
[0049] The layer 11 is made of a material presenting a high
refractive index n.sub.h. This index is preferably greater than or
equal to (5/3)n.sub.max, where n.sub.max is the highest refractive
index of the surrounding materials.
[0050] In the example shown in FIG. 1, the layer 11 does not rest
on any substrate. Thus, the medium surrounding the filter 1 is
air.
[0051] This embodiment is illustrated in particular in the
following articles: "Guided mode resonance in sub-wavelength
metallodielectric freestanding grating for bandpass filtering" by
E. Sakat, G. Vincent, P. Ghenuche, N. Bardou, S. Collin, F. Pardo,
J.-L. Pelouard, R. Haidar, Opt. Lett. 36, 3054 (2011), and
"Freestanding sub-wavelength metallic gratings for snapshot
multispectral imaging" by R. Haidar, G. Vincent, S. Collin, N.
Bardou, N. Guerineau, J. Deschamps, J.-L. Pelouard, Appl. Phys.
Lett. 96, 221104 (2010).
[0052] The height h.sub.h of the layer 11 preferably lies in the
range approximately 0.85.lamda./2n.sub.h and approximately
1.5.lamda./2n.sub.h, where .lamda. is a wavelength in the
predetermined range of wavelengths.
[0053] In another embodiment, the lower surrounding medium may
constitute a substrate. It may be made of a material having a
refractive index that is less than the refractive index of the
material of the layer 11. If it is made of material presenting an
index close to n.sub.h, or even higher, then a quarterwave layer
made of a material having an index lower than n.sub.h should be
interposed between the filter 1 and the substrate.
[0054] Inclusions 111 are provided in this layer 11, the inclusions
being made of a material having a refractive index n.sub.b that is
strictly less than the index n.sub.h of the layer 11.
[0055] The difference between the indices n.sub.h and n.sub.b is
preferably greater than or equal to 0.25n.sub.h.
[0056] In general, it is found that the rejection of the filter is
improved over a broad spectral range with increasing difference
between the indices n.sub.h and n.sub.b.
[0057] Thus, the layer 11 is structured by replacing the material
having a high refractive index n.sub.h in the zones 111 of the
layer 11 with a material having a low refractive index n.sub.b.
[0058] When structured in this way, the layer 11 conserves a
waveguide function, which function is adjusted by the inclusions
ill so as to improve filtering performance.
[0059] These inclusions ill extend to the surface 112 of the layer
11 that is opposite from the face 110 that supports the reflective
elements 10. Thus, in this embodiment, the inclusions 111 are flush
with this surface 112.
[0060] Tests have been performed that show that this embodiment is
the most favorable in terms of rejection. Nevertheless, it is
possible to envisage the presence of a layer of material on the
surface 112, in particular if its thickness is less than
0.05h.sub.h.
[0061] That is why, in the context of the present patent
application, the term "flush" extends to include both inclusions
that do indeed reach the surface 112 and also inclusions that are
separated from this outside surface of the layer 11 by a fine layer
of material.
[0062] In the example shown in FIG. 1, these inclusions 111 are
distributed in the layer 11 at a period p equal to P.
[0063] Other embodiments could be envisaged.
[0064] In this example, the inclusions 111 are in the form of
continuous strips.
[0065] The materials constituting the layer 11 and the inclusions
111 are materials that are dielectric in the optical meaning of the
term, i.e. they absorb little or no light at the wavelength of
interest.
[0066] The height h.sub.b of the inclusions 111 lies in the range
0.5h.sub.h to h.sub.h or advantageously in the range 0.5h.sub.h to
0.95h.sub.h. Thus, the height h.sub.b of the inclusions may be
equal to the height h.sub.h of the layer 11. Under such
circumstances, the layer 11 is made up of a periodic alternation
between strips of material having a refractive index n.sub.h and
strips of material having a refractive index n.sub.b, these strips
all having the same thickness.
[0067] Furthermore, the width l.sub.b of the inclusions 111 lies in
the range 0.05P to 0.75P.
[0068] In combination, these two ranges of values for h.sub.b and
l.sub.b serve to improve rejection outside the transmission range
while having good transmission in the desired range.
[0069] Preferably, the height h.sub.b and the width l.sub.b of the
inclusions 111 are selected as follows.
[0070] The height h.sub.b of the inclusions 111 should lie in the
range 0.7h.sub.h and 0.95h.sub.h, and in particular in the range
0.8h.sub.h to 0.9h.sub.h, for example it may be equal to about
0.85h.sub.h.
[0071] The width l.sub.b of the inclusions preferably lies in the
range 0.1P to 0.7P, and more particularly in the range 0.15P to
0.55P. Typical values that are used may be about 0.3P or 0.5P.
[0072] Rejection is thus further improved outside the transmission
range without transmission being degraded in the transmission
range, when the ranges of values for h.sub.b and l.sub.b are
reduced around the above-mentioned optimum values.
[0073] It should also be observed that an inclusion is not
positioned arbitrarily relative to the reflective elements 10. Each
inclusion 111 is positioned so as to be situated at least in part
between two reflective elements. Naturally, this positioning is
considered in the thickness of the filter, i.e. the inclusions 111
are indeed situated below the reflective elements 10, but they are
at least in part in register with a slit 100.
[0074] In general, the reference positioning between the slits and
the inclusions is mutual centering between them.
[0075] Nevertheless, a certain amount of tolerance exists for this
relative positioning.
[0076] Thus, the filter of the invention is not restricted to the
embodiment shown in FIG. 1, and each inclusion 111 could be offset
so as to be in register only in part with a slit 100.
[0077] Tests have shown that the inclusions shown in FIG. 1 can be
offset by about 10% of the period P without the rejection of the
filter being greatly disturbed. This is an offset between the
center of an inclusion and the center of a slit. These tests have
been performed in particular on a filter of the invention as
described below with reference to FIG. 3.
[0078] This value is quite large and makes fabrication robust in
the face of the alignment errors that are commonly encountered in
microfabrication (typically, ultraviolet (UV) lithography presents
an alignment error of a few tens of nm).
[0079] By way of example, the following materials could be
used.
[0080] When the optical filter of the invention is for a wavelength
situated in the visible, the material of high index n.sub.h may for
example be made of Si.sub.3N.sub.4, SiC, CdS, or indeed ZnS. The
material of low index n.sub.b of the inclusions 111 may then be
SiO.sub.2, CaF.sub.2, or indeed SiON.
[0081] Thus, in the visible, the minimum value of the index n.sub.h
of the high index materials is about 2 (as for Si.sub.3N.sub.4),
and the maximum value of the index n.sub.b of the low index
materials is about 1.5 (as for SiO.sub.2). This does indeed
correspond to a difference between n.sub.h and n.sub.b that is
greater than or equal to 0.25n.sub.h.
[0082] When the filter of the invention is for wavelengths situated
in the infrared, the material of high index n.sub.h may be selected
from semiconductor materials having an optical cutoff wavelength
that is shorter than the wavelength of interest. For example, the
material may be silicon for wavelengths longer than 1.1 .mu.m or
germanium for wavelengths longer than 1.5 .mu.m. Other high index
semiconductor materials of type II-V or of type II-VI may also be
suitable. In this respect, it should be observed that semiconductor
materials presenting a small gap present refractive indices that
are quite high, of the order of 3 to 4.
[0083] Furthermore, the above-mentioned high-index materials for a
filter for use at a wavelength situated in the visible are also
suitable for a filter for use with a wavelength situated in the
infrared.
[0084] The materials of low refractive index n.sub.b may for
example be SiO.sub.2, MgO, Al2.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2,
or indeed ZnO, or indeed nitrides or sulfides.
[0085] In preferred manner, for wavelengths of interest shorter
than 8 .mu.m, it is advantageous to select a layer 11 made of
silicon with inclusions made of SiO.sub.2. This combination of
materials makes it possible to make filters at low cost, using
integrated fabrication on a technological platform making use of
equipment for deposition, lithography, etching, cleaning, and
annealing devices presenting patterns of micrometer and/or
submicrometer size.
[0086] Nevertheless, for wavelengths of interest situated in the
range 8 .mu.m to 12 .mu.m, it is advantageous to select germanium
for the material of the layer 11 and ZnS for the inclusions.
Silicon oxide is a material that is highly absorbent at a
wavelength of about 8 .mu.m, which makes it preferable to use some
other material for wavelengths longer than 8 .mu.m, and in
particular in the 8 .mu.m-12 .mu.m band (known as band III, or the
far infrared). In general, the situation is similar for all oxides
and nitrides. In contrast, sulfides make it possible to overcome
this difficulty, even if they present a refractive index that is
higher. It should be observed that sulfides may be used as a
material of high index in the visible, or as a material of low
index in the infrared.
[0087] In general, the difference between the indices n.sub.b and
n.sub.h will be smaller in the visible than in the infrared.
[0088] This comes from the difficulty of finding a material of high
index (in particular higher than 3) that is also transparent in the
visible range, in combination with the fact that materials of low
index do not change relative to the IR range (typically SiO.sub.2
with an index of 1.5).
[0089] Rejection will therefore be weaker in the visible.
[0090] A method of making a second embodiment of the optical filter
of the invention is described below with reference to FIGS. 2a to
2f.
[0091] FIG. 2a shows a first step of the method in which a layer of
silicon oxide 3 is deposited on a silicon substrate 2.
[0092] This deposition may be performed in particular using plasma
enhanced chemical vapor deposition (PECVD) or thermal oxidation at
high temperature (about 1000.degree. C.).
[0093] FIG. 2b shows a second step of the method in which a layer 4
of silicon is deposited on the layer 3, e.g. by PECVD.
[0094] FIG. 2c shows a step of lithography followed by a step of
etching to etch the layer 4 locally, this etching being stopped by
the layer 3.
[0095] This etching step, e.g. reactive plasma etching, serves to
provide empty zones 40 between zones 41 of silicon.
[0096] These zones 40 are distributed periodically, at a period
p.
[0097] The lithography technique that is used is preferably a deep
ultraviolet (DW) technique.
[0098] FIG. 2d shows a step of the method in which a layer of
SiO.sub.2 is deposited on the layer 3 in such a manner as to
encapsulate the remaining material of the layer 4.
[0099] The thickness of the deposited layer of SiO.sub.2 is greater
than the height of the zones 40 or 41. After it has been deposited,
a step of mechanical-chemical polishing is performed so that the
resulting layer presents the same height as the zones 41.
[0100] The layer obtained on the layer 3 thus has alternating zones
42 made of SiO.sub.2 and zones 41 made of silicon.
[0101] FIG. 2e shows two other steps of the method of the
invention.
[0102] The first consists in depositing an additional layer of
silicon on the zones 40 and 41.
[0103] This step serves to obtain a silicon layer 11 in which
inclusions 111 of SiO.sub.2 are present.
[0104] This additional layer of silicon may be deposited in
particular by a PECVD type technique.
[0105] The second step consists in depositing a metal layer 5 on
the layer 11, in particular a layer of aluminum.
[0106] Under certain circumstances, another layer (not shown in
FIG. 2e) may be deposited between the layer 11 and the layer 5.
This layer has a diffusion barrier function. This layer may be
useful if heat treatment is performed after depositing the metal
layer 5 leading to interdiffusion at the interface between the
layers 5 and 11.
[0107] This barrier layer may in particular be made of silicon
nitride.
[0108] FIG. 2f shows a last step of the method of the invention,
which consists in a step of lithography followed by etching the
layer 5, e.g. reactive plasma etching.
[0109] This etching step leads to obtaining reflective elements 10
on the face 110 of the layer 11.
[0110] Thus, the resulting filter 20 differs from the filter 1
shown in FIG. 1 in that it includes a silicon substrate 2, with a
layer 3 of silicon oxide being provided between the layer 11 and
the substrate 2.
[0111] The performance of a filter of the type shown in Document FR
2 959 021 with a filter of the invention, of the same type as the
optical filter shown in FIG. 2f is compared below with reference to
FIGS. 3 to 5.
[0112] Thus, FIG. 3 has two curves C.sub.1 and C.sub.2, the curve
C.sub.1 showing the transmission response of an optical filter of
the invention as a function of wavelength, while the curve C.sub.2
is a similar curve for an optical filter in accordance with
Document FR 2 959 021.
[0113] Both of those filters had reflective elements defining a
periodic grating of parallel slits, of period P, of height h, and
of width l. The reflective elements were made of aluminum and were
in the form of parallel strips.
[0114] The height h of the slits was 50 nm. The period P of the
filter of the invention was 2000 nm, while the period of the prior
art filter was 1700 nm. In both cases, the width l of the slits was
selected to be equal to 0.2P. That led to l having a value equal to
400 nm for the filter of the invention and to 340 nm for the prior
art filter.
[0115] The reflective elements were formed on a support layer of
height equal to 575 nm.
[0116] In both filters, the support layer was arranged on a layer
of SiO.sub.2 presenting a height of 700 nm, the layer of SiO.sub.2
itself being arranged on a silicon substrate.
[0117] In the prior art filter, the support layer was made of
silicon only.
[0118] In the filter of the invention, the support layer was made
of silicon and it also included inclusions of SiO.sub.2. The
inclusions were distributed in the layer 11 at a period p equal to
2000 nm, each of the inclusions being situated in register with a
slit between two reflective elements.
[0119] The height h.sub.b of the inclusions was selected to be
equal to 0.8h.sub.h, where h.sub.h was the height of the support
layer, corresponding to a value of 460 nm. The width l.sub.b of the
inclusions was selected to be equal to 0.2P, corresponding to a
value of 400 nm.
[0120] As can be seen in FIG. 3, both filters had a resonance
wavelength of 4 .mu.m.
[0121] Furthermore, it can be seen from FIG. 3 that, in curve
C.sub.2, the width of the filter spectral band is 0.75.lamda. for a
rejection ratio of 10 decibels (dB) and of 0.9.lamda. for a
rejection ratio of 5 dB.
[0122] For the curve C.sub.1, the width of the filter spectral band
is 1.70.lamda. for a rejection ratio of 10 dB, and it is equal to
about 2.lamda. for a rejection ratio of 5 dB.
[0123] Thus, FIG. 3 shows that the filter of the invention makes it
possible to substantially improve the rejection spectral width
since it is doubled, in particular in the long wavelength range,
i.e. wavelengths longer than the resonance wavelength of the
filter.
[0124] FIG. 4 shows two curves T.sub.1 and T.sub.2, the curve
T.sub.1 showing the transmission response of an optical filter of
the invention as a function of wavelength, while the curve T.sub.2
is a similar curve for an optical filter in accordance with
Document FR 2 959 021.
[0125] Those two filters had the same structures as the filters
described with reference to FIG. 3, except for a few
characteristics as specified below.
[0126] Thus, the filter in accordance with Document FR 2 959 021
was identical to that corresponding to curve C.sub.2. The curves
C.sub.2 and T.sub.2 are identical.
[0127] The period P of the optical filter of the invention
presented reflective elements of aluminum in the form of strips
distributed at a period P equal to 1850 nm. The width l of the
slits was still 0.2P, thus corresponding to a value of 370 nm.
[0128] Furthermore, the height h.sub.h of the layers supporting the
reflective elements and made of silicon was equal to 725 nm.
[0129] The inclusions of SiO.sub.2 had the configuration of FIG. 1.
Their height h.sub.b was equal to 0.8h.sub.h, corresponding to a
value of 580 nm. The width l.sub.bof the inclusions was equal to
0.3P, which corresponds to a value of 555 nm.
[0130] It can be seen that the height of the support layer, equal
to 575 nm in the prior art filter, corresponds substantially to a
value of 0.99.lamda./2n.sub.h. For the filter of the invention, the
height h.sub.h of 725 nm corresponds to a value of
1.24.lamda./2n.sub.h.
[0131] On comparing the curves T.sub.1 and T.sub.2, it can be seen
that the filter of the invention still presents good rejection over
a larger spectral band than does the prior art filter.
[0132] Thus, the curve T.sub.1 shows that the filter of the
invention presents rejection of at least 13.7 dB over a filter
spectral band of about 0.74.lamda., while the corresponding band
for the curve T.sub.2 is about 1.73.lamda..
[0133] Furthermore, if the curves C.sub.1 and T.sub.1 are compared,
it can be seen that the increase in the height h.sub.h of the
support layer also makes it possible to increase the width of the
spectral band over which a given level of rejection is
obtained.
[0134] The table below gives a comparison between each of the two
filters of the invention corresponding to curves C.sub.1 and
T.sub.1 and the prior art filter corresponding to curves C.sub.2
and T.sub.2, the table giving the spectral band width for rejection
ratios of 5 dB, 10 dB, and 13.7 dB.
TABLE-US-00001 C.sub.1 T.sub.1 Prior art 5 dB 2.71.lamda.
2.73.lamda. 0.93.lamda. 10 dB 1.72.lamda. 1.82.lamda. 0.8.lamda.
13.7 dB 1.07.lamda. 1.73.lamda. 0.73.lamda.
[0135] Furthermore, comparing curves C.sub.1 and T.sub.1 shows that
the increase in the width of the filter spectral band for a given
rejection ratio that results from increasing the height of the
support layer is obtained while conserving the properties of the
main transmission peak at the resonance wavelength, i.e. 4 .mu.m.
In particular, the maximum transmission value and the half-height
width in curves C.sub.1 and T.sub.1 are substantially
unchanged.
[0136] However, comparing curves C.sub.1 and T.sub.1 shows that the
maximum transmission value for the secondary peak situated at about
2.425 nm is greater for the curve T.sub.1 than for the curve
C.sub.1. This maximum value is about 0.53 for the curve T.sub.1 and
0.27 for the curve C.sub.1.
[0137] A selection between the two types of filter can be made as a
function of the particular application that is envisaged.
[0138] FIG. 5 has two curves G.sub.1 and G.sub.2, the curve G.sub.1
showing the transmission response of an optical filter of the
invention as a function of wavelength, while the curve G.sub.2 is a
similar curve for an optical filter in accordance with Document FR
2 959 021.
[0139] Those two filters had reflective elements made of aluminum
separated by a grating of slits of period P, of height h, and of
width l.
[0140] In both filters, the period P was selected so that the value
of the resonance wavelength was equal to 10 .mu.m.
[0141] For the filter of the invention, P was equal to 3350 nm
whereas for the filter in accordance with Document FR 2 959 021,
the value of P was equal to 3570 nm. For both of them, the height h
of the slits was 50 nm and their width was 0.2P. That led to a
value for l equal to 670 nm for the filter of the invention and to
714 nm for the filter of the prior art.
[0142] In both filters, the layer supporting the reflective
elements was placed on a layer of ZnS presenting a height of 1125
nm, the ZnS layer itself being supported by a silicon
substrate.
[0143] For the prior art filter, the support layer was made of
germanium and it had a height of 1250 nm.
[0144] For the optical filter of the invention, the support layer
had a height h.sub.h of 1500 nm. It was made of germanium with
inclusions of ZnS. The configuration of those inclusions was as
shown in FIG. 1.
[0145] The height h.sub.b of the inclusions was equal to
0.85h.sub.h, corresponding to a value of 1275 nm.
[0146] The width l.sub.b of the inclusions was equal to 0.3P,
corresponding to a value of 1005 nm.
[0147] FIG. 5 shows that for given rejection, e.g. about 8 dB, the
corresponding spectral band presents a greater width for the curve
G.sub.1 than for the curve G.sub.2 (a value of 1.06.lamda., as
compared with 0.76.lamda.).
[0148] Furthermore, tests were performed to determine the influence
of the angle of incidence of the light on the filter.
[0149] Those tests were performed using the filters corresponding
to curves T.sub.1 and T.sub.2.
[0150] They reveal that both filters had the same lack of
sensitivity to angle of incidence. Thus, for angles of incidence
extending as far as 10.degree., the peak corresponding to the
maximum transmission value was always obtained for a wavelength
value of 4 .mu.m.
[0151] Furthermore, those tests show that the width extension of
the spectral band obtained with the filter of the invention for
given rejection is insensitive to the angle of incidence of light
to a value of 10.degree..
[0152] The curves C.sub.1, T.sub.1, and G.sub.1 shown in FIGS. 3 to
5 show that for wavelengths longer than 10 .mu.m for the curves
C.sub.1 and T.sub.1 and longer than 14 .mu.m for the curve G.sub.1,
the transmission values increased significantly.
[0153] To limit that phenomenon, it is possible to envisage doping
the semiconductor material used for fabricating the filter, and
more particularly the top layer of the substrate.
[0154] Thus, FIG. 6 has two curves H.sub.1 and H.sub.2. The curve
H.sub.1 shows the transmission response of the optical filter of
the invention corresponding to the curve T.sub.1. Thus, the curves
H.sub.1 and T.sub.1 are identical. The curve H.sub.2 corresponds to
the same filter, in which a plate made of n-doped silicon was
inserted between the silicon substrate and the layer of SiO.sub.2.
In practice, that plate corresponds to the top portion of the
substrate being doped.
[0155] The plate had a height of 2 .mu.m. Comparing the curves
H.sub.1 and H.sub.2 shows that the plate serves to limit
transmission strongly at wavelengths longer than 9 .mu.m.
[0156] It can also be seen that inserting the plate of doped
silicon leads to a decrease in the value of the main transmission
peak at the wavelength of 4 .mu.m. However this decrease is only
20%. It is therefore acceptable in terms of proper operation of the
filter, given the limit on transmission for long wavelengths.
[0157] Reference is made below to FIG. 7 which, for an optical
filter of the invention, shows the maximum value of a cost function
F as a function of the height h.sub.b of the inclusions (plotted
along the abscissa axis) and of their width l.sub.b (plotted up the
ordinate axis).
[0158] The cost function is defined as follows:
F ( h b , l b ) = T 35 2 ( h b , l b ) T 23 ( h b , l b ) T 57 ( h
b , l b ) ##EQU00002##
where T.sub.35 (h.sub.b, l.sub.b), T.sub.23(h.sub.b, l.sub.b), and
T.sub.57(h.sub.b, l.sub.b) correspond to the maximum transmission
of a filter having the characteristics given below in the following
spectral bands respectively [3 .mu.m to 5 .mu.m]; [2 .mu.m to 3
.mu.m]; and [5 .mu.m to 7 .mu.m]. Given the way this cost function
F is defined, it is at a maximum for a given inclusion dimension
when there is strong rejection away from the main transmission peak
appearing in the [3 .mu.m to 5 .mu.m] spectral band. Thus, this
cost function reaches a value greater than 50 for h.sub.b=475 nm
and l.sub.b=500 nm.
[0159] FIG. 7 was obtained for a filter of the invention having
reflective elements made of gold and separated by a grating of
parallel lines. The period P of the grating was 1760 nm, the width
l of the slits was 360 nm, and the height of the slits was 50
nm.
[0160] The optical filter had a support layer of height h.sub.h
equal to 575 nm. The support layer was made of silicon and
presented SiO.sub.2 inclusions of width l.sub.b and height
h.sub.b.
[0161] The support layer was deposited on a layer of SiO.sub.2
presenting a height of 700 nm, itself deposited on a substrate of
silicon.
[0162] Including the inclusions serves to vary the resonance
wavelength of the filter, which depends on the values of h.sub.b
and l.sub.b. Thus, FIG. 8 shows the value of the resonance
wavelength as a function of h.sub.b and of l.sub.b.
[0163] FIG. 7 shows that good rejection is obtained for a value of
h.sub.b lying in the range 0.5h.sub.h to h.sub.h or indeed in the
range 0.5h.sub.h to 0.95h.sub.h and for values of l.sub.b lying in
the range 0.05P to 0.75P. Rejection is optimized for a value of
l.sub.b equal to 0.28P and a value of h.sub.b equal to 0.8h.sub.h.
These values are compatible with those mentioned above, given that
they are obtained by simulation.
[0164] Thus, the optical filter of the invention presents numerous
advantages over optical filters of the prior art.
[0165] Firstly, it presents very good rejection over a very wide
spectral range.
[0166] The optical filter of the invention may include a support
layer made of a combination of Si and of SiO.sub.2, in particular
when the filter is for use at wavelengths situated in the range 1
.mu.m to 8 .mu.m. The optical filter can then be made easily using
complementary metal oxide semiconductor (CMOS) technology, thereby
enabling its fabrication costs to be reduced.
[0167] Optical filters of the invention also provide good
transmission, typically higher than 70%, over a narrow spectral
range.
[0168] They present good angular insensitivity concerning the
position of the resonance wavelength and the transmission of the
filters, to an angle of about 10.degree..
[0169] Finally, filters of the invention are selective in
transmission (bandpass) and in reflection (band stop) and they are
compact.
[0170] The reference signs inserted after the technical
characteristics that appear in the claims are provided solely to
facilitate understanding of the claims and not to limit their
scope.
* * * * *